This section provides background information related to the present disclosure which is not necessarily prior art.
Thrombosis is the medical term for the process of pathologic blood clot formation—the key mechanism underlying many cardiovascular diseases, including stroke, myocardial infarction, deep vein thrombosis (DVT), etc. These thrombi can break off from site of formation and travel to distant sites (embolisation) and cause symptoms at sites distinct from the site of formation. Further these processes may manifest in conduits that are placed in the vascular bed to bypass blood flow (eg grafts) or as extensions to the vascular bed (eg drive lines for cardiac assist devices, implantable venous catheters etc). Each of these conditions poses a significant clinical problem. For example, DVT is the formation of blood clots in the deep veins, most commonly those of the lower legs. DVT has an incidence rate of 1 in 1000 persons. Up to 5% of patients with DVT experience pulmonary embolism, which causes at least 100,000 deaths annually in the United States.
To treat thrombosis, the pathologic blood clot (thrombus) or clot fragment (embolus) needs to be removed. Current clinical treatments to remove thrombi include thrombolytic drugs, catheter-based surgical procedures, and direct surgical removal of clots. Treatment of thrombosis usually encompasse either breakup of the clot (thrombolysis) or removal (thrombectomy). These terms are used in reference to both thrombus and emboli irrespective of site of formation or disease and are used herein as such.
Thrombolytic drugs (e.g, rt-PA) dissolve the blood clot by breaking down the cross-linked fibrin structures that solidify the clot. Thrombolytic drugs systemically stimulate the fibrinolytic process while suppressing the anti-fibrinolytic process. Therefore, both thrombosis and normal hemostatic clot formation (vessel wound healing) are inhibited. Inhibition of normal hemostatic clot formation is associated with an increase in bleeding complications, which may be fatal in a small number of cases.
In contrast, treatments using catheter-based devices are localized to the target clot. The current catheter-based thrombolysis procedures include catheter-based local delivery of thrombolytic agents, vein segment isolation and thrombolysis, and mechanical disruption and aspiration of the clot (Rheolytic thrombectomy). However, catheter-based devices are invasive and carry an increased risk of bleeding, damage to the vessel wall, and infection. In rare cases, catheter-based thrombolysis methods may also result in death.
Direct surgical methods are even more invasive than catheter-based methods. Clinicians make a small incision through the skin and surgically remove the clot directly.
Researchers have been exploring new means to improve the efficiency and safety of thrombosis treatment techniques. Minimally invasive or non-invasive ultrasound methods to treat thrombosis have been developed.
Studies have shown that ultrasound energy can accelerate thrombolysis by facilitating the delivery of thrombolytic drugs to the target clot. Thrombolysis refers to dissolving or breaking up of a thrombus. For example, ultrasound combined with rt-PA can dissolve a clot within 30 minutes, which would otherwise take 3 hours using rt-PA alone. Ultrasound energy can be generated by inside the vessel through a catheter-based transducer (Rosenscehin et al, U.S. Pat. No. 5,163,421, Tachibana et al U.S. Pat. No. 6,001,069) or outside the patent body through an external transducer non-invasively (Holland et al., U.S. Pat. No. 7,300,414). Even though this method increases thrombolysis efficiency, it still carries the undesired side effects of thrombolytic drugs. This hybrid technique is still being studied and not currently in clinical use.
Recently, some researchers have been investigating the possibility of achieving thrombolysis using ultrasound alone or combined with contrast agents, without the use of pharmaceutical drugs. Using microbubbles induced by high intensity focused ultrasound (Rosenschein et al. U.S. Pat. Nos. 5,524,620 and 6,113,558) or via injected contrast agents (Unger et al, U.S. Pat. No. 6,576,220, Siegel et al, U.S. Pat. No. 5,695,460), blood clot removal can be achieved. Similarly, ultrasound energy can be generated inside the vessel or outside the patient body. However, the mechanism is not well understood, and therefore, these techniques remain far from clinical application.
Acoustic cavitation has been claimed to be a possible mechanism of some older ultrasound thrombolysis methods. Acoustic cavitation is a term used to define the interaction of an acoustic field, such as an ultrasound field, with bodies containing gas and/or vapor. This term is used in reference to the production of small cavities, or microbubbles, in the liquid. Specifically, when an acoustic field is propagated into a fluid, the stress induced by the negative pressure produced can cause the liquid to rupture, forming a void in the fluid which may contain vapor and/or gas. Acoustic cavitation also refers to the oscillation and/or collapse of microbubbles in response to the applied stress of the acoustic field. However, no one has previously succeeded in achieving controlled and predictable cavitation for thrombolysis with real-time ultrasound feedback.
Methods have been developed to initiate, maintain, and control cavitation for use in general therapy. For example, Cain et al. (U.S. Pat. No. 6,309,355), which is hereby incorporated by reference, describes apparatus and methods that use cavitation induced by an ultrasound beam to create a controlled surgical lesion in a selected therapy volume of a patient.
As indicated, previous ultrasound thrombolysis methods involve the use of thrombolytic drugs or microbubbles. Other methods that use ultrasound energy alone, invasive methods or even noninvasive methods, do not allow easy assessment or feedback of when the process is operating effectively, and often do not provide any feedback which can be used to optimize the process. Consequently, more effective methods and techniques for ultrasound thrombolysis therapies are desirable and would enable beneficial noninvasive alternatives to many present methods in the thrombosis treatment field. In particular, monitoring treatment and receiving feedback during the procedure would inform a clinician whether the procedure is progressing adequately according to plan and when it can be ended. As such, the ability to monitor and adjust the ultrasound thrombolysis therapy concomitant with treatment would provide significant advantages over prior ultrasound thrombolysis therapies.